This invention relates to an article formed of a tubular sheath filled with powder and, more particularly, to a welding filler metal having a net metallic composition of a high-gamma-prime nickel-base superalloy.
In a form of welding, a metallic article to be welded is locally melted, and the melted metal is mixed with a second metal. The temperature is thereafter reduced so that the melted material solidifies. In one approach, the second metal is another article, so that the two articles are joined together. In another approach, the second metal is an overlay deposit that is also melted during the welding process, with the result that the first article is overlaid with the second metal.
A filler metal may be used in either of these approaches. In the joining of two articles by welding, the filler metal may be added into the melted zone to fill the space between the two articles. In the overlay process, the filler metal may form substantially the entire overlay. The filler metal may be the same as one or both of the articles being joined in the first approach. In the second approach, the filler metal may be the same as the article being overlaid, such as when the dimensions of the article are being restored during a repair process, or of a different composition to provide particular properties to the surface of the overlaid article.
The filler metal is often supplied as a weld wire that is used in automated welding apparatus and other welding procedures. (As used herein, xe2x80x9cwirexe2x80x9d and xe2x80x9cweld wirexe2x80x9d include physical forms that are considered rods and also physical forms that are considered wires, avoiding the need for any arbitrarily selected distinction as to whether the physical form is a rod or a wire.) A heat source, such as an electrical welding power supply or a beam source such as a laser or electron beam, heats the region of the article to be melted, forming a molten pool. The weld-wire filler metal is gradually fed into the molten pool to supply the desired volume of the filler metal.
The welding filler metal may be produced in weld-wire form in various ways. In one approach, it is cast as a billet and then extruded or wire drawn to smaller transverse size. In another approach, it is consolidated as a powder into a billet, and then extruded or wire drawn to smaller transverse size. In either of these fabrication techniques, the extruded article is centerless ground to achieve the desired shape and size, and to remove the remnants of the extrusion operation.
Some alloys of interest as filler metals in welding applications, notably nickel-base alloys such as nickel-base superalloys with a high volume fraction of gamma prime phase when heat treated, cannot be wire drawn due to their work hardening properties and limited ductilities. The welding filler metal weld wire is therefore produced by a specialized extrusion process, followed by acid etching and centerless grinding of the extruded material. As a result, the manufacturing yields of usable weld wire are low, typically about 25 percent of the weight of the starting material. The process is also relatively expensive. The cost of the weld wire is therefore high, relative to the material cost.
There is a need for an improved approach to the fabrication of weld wire and related types of products made of difficult-to-draw nickel-base alloys such as high-gamma-prime nickel-base superalloys. The present invention fulfills this need, and further provides related advantages.
The present approach provides a powder-cored article that is useful as the filler metal in welding. The article has a nickel-based net metallic composition according to the compositions and relative amounts of its components. When used as a welding wire, the powder-cored article used in the same manner as conventional weld wire, and is melted into the molten weld pool to provide the desired net metallic composition. The powder-cored article is relatively inexpensive to manufacture, with a high yield of finished material. With the present approach, there is essentially a 100 percent yield of the usable weld wire, as compared with the weight of the starting material. The cost of the final product is less than the cost of the welding wire of the same net composition prepared by the conventional extrusion approach.
In accordance with the invention, an article has a net metallic composition and comprises a tubular sheath made of a sheath nickel-base alloy, and a core disposed within the tubular sheath and comprising powder particles made of a core nickel-base alloy. Initially, the powder particles are packed relatively loosely, but during processing they are compacted so that they have substantially no voids therebetween. Preferably, the ratio of the weight of the core nickel-base alloy to the weight of the total of the core nickel-base alloy plus the sheath nickel-base alloy is not greater than about 0.3.
Preferably, the sheath nickel-base alloy has no more than about 4 percent of aluminum by weight, and the core nickel-base alloy has more than about 4 percent of aluminum by weight. In one embodiment, the sheath nickel-base alloy has from 0 to about 4 percent by weight aluminum, and the core nickel-base alloy has from about 10 to about 15 percent by weight aluminum. The sheath nickel-base alloy has a sheath-nickel-base-alloy liquidus temperature and the core-nickel-base alloy has a core-nickel-base-alloy liquidus temperature. It is preferred that the sheath-nickel-base-alloy liquidus temperature and the core-nickel-base alloy liquidus temperature are sufficiently close that they melt substantially simultaneously, which is typically achieved if they are within about 200xc2x0 F. of each other. It is also preferred that the sheath-nickel-base-alloy liquidus temperature is less than the core-nickel-base-alloy liquidus temperature.
The net metallic composition of the article is desirably a nickel-base superalloy hardenable by the precipitation of gamma prime or a related phase. More preferably, the net composition of the article is desirably a high-gamma-prime nickel-base superalloy having more than about 30 volume percent gamma prime phase upon heat treatment. In a preferred embodiment, the sheath nickel-base alloy has a composition in weight percent of about 12 percent cobalt, about 1.5 percent molybdenum, about 4 percent tungsten, about 3.5 percent aluminum, about 4 percent tantalum, about 3.75 percent rhenium, about 8 percent chromium, about 2 percent hafnium, about 0.004 percent boron, about 0.002 percent carbon, balance nickel and minor elements, and the core nickel-base alloy has a composition in weight percent of about 12 percent cobalt, 1.5 percent molybdenum, about 7.6 percent tungsten, about 14.2 percent aluminum, about 13.5 percent tantalum, about 8 percent chromium, about 0.04 percent yttrium, about 0.05 percent boron, about 0.41 percent carbon, balance nickel and minor elements.
The core may also include nonmetallic particles, such as nonmetallic reinforcement particles or fibers, mixed with the metallic powder. Examples include oxide particles. The nonmetallic particles are not melted during any welding operation. The composition of the nonmetallic particles, if any, is not included in the compositional calculations of the net metallic composition.
A method of making an article comprises the steps of preparing a preform having a net metallic composition and comprising a tubular sheath made of a sheath nickel-base alloy, and a core disposed within the tubular sheath and comprising consolidated powder particles made of a core nickel-base alloy. The preform is wire drawn to produce a drawn article. The drawn article may be of any diameter, but is typically less than about 0.125 inches in outside diameter. The drawn article is preferably used as a weld filler material, as by melting the drawn article to form a melted article during the welding operation. Compatible features discussed elsewhere herein may be used in conjunction with the method.
When the present approach is used in its preferred application of making a nickel-base superalloy weld wire, care is taken to allocate the aluminum between the sheath and the core. Nickel-base superalloys typically have from about 3 to about 8 percent by weight aluminum. The sheath nickel-base alloy has no more than about 4 percent of aluminum by weight, with the balance of the aluminum provided by the core nickel-base alloy. If the sheath nickel-base alloy has more than about 4 percent of aluminum by weight, it is difficult or impossible to reduce the article by wire drawing due to the work hardening characteristics of the sheath nickel-base alloy. Thus, by placing the higher aluminum content into the powdered core and having a relatively lower aluminum content in the sheath, the combination can be wire drawn yet still have the desired nickel-base superalloy net composition.
The result of using this approach is that the cost of the welding wire is substantially reduced. When articles such as high-gamma-prime nickel-base superalloy welding wire are produced by extrusion, a cast rod of the final desired composition is enclosed in an extrusion can, the extrusion can with its enclosed rod is extruded, the can material is chemically etched away, and the remaining extruded material is centerless ground to the desired size. There is a low yield of usable welding wire, typically in the range of about 25 percent of the weight of the starting material, achieved with high production costs.
With the present approach, care is taken so that the liquidus temperatures of the sheath and the core materials are relatively close together, with the sheath-nickel-base-alloy liquidus temperature preferably slightly lower than the core-nickel-base-alloy liquidus temperature. The core and the sheath melt at about the same temperature and time, but with the sheath melting slightly before the core.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.